1. Field of the Invention
Exemplary aspects of the present invention generally relate to a drive unit and an image forming apparatus including the drive unit, and more particularly to an image forming apparatus such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Such known image forming apparatuses employ a drive unit to drive various components. An example of a known drive unit includes a drive timing belt entrained around a drive pulley provided to a drive shaft of a drive motor and a relay pulley, and a driven timing belt entrained around the relay pulley and a driven pulley, thereby transmitting the driving force of the drive shaft of the drive motor to the rotary shaft of targets to be moved. Such targets include, but are not limited to, the image bearing member (i.e., a photosensitive drum), the intermediate transfer member, and a recording medium conveyor.
In the known drive unit, the drive timing belt and the driven timing belt are arranged in a rotary shaft direction of the relay pulley. In order to reduce a cyclic speed variation of the drive timing belt and the driven timing belt in one rotation period thereof, the drive timing belt and the driven timing belt are arranged such that eccentric components of these belts counteract each other on the driven pulley. More specifically, as the eccentric component of the drive timing belt is transmitted to the driven timing belt via the relay pulley, waves of the eccentric component of the drive timing belt cancel waves of the eccentric component of the driven timing belt. Accordingly, the cyclic speed variation in one rotation period of each timing belt is not transmitted to the driven pulley, thereby preventing fluctuation in the speed of targets.
The drive unit employed in the image forming apparatus needs to rotate the targets at a certain speed. Thus, a rotation force of the drive motor serving as a drive source of the drive unit is adjusted in accordance with a rotation load so as to rotate the targets at a desired constant speed. However, if there is a sudden change in the load, unless the tension of the drive timing belt and the driven timing belt is optimized, the drive timing belt and the driven timing belt cannot rotate in accordance with the force transmitted from the drive motor, causing fluctuation in the speed of devices.
In view of the above, there is unsolved need for a drive unit capable of preventing fluctuation of speed of devices and a cyclic speed variation in one rotation of timing belts, and an image forming apparatus including such a drive unit.